1. Field of the Invention
The present invention relates generally to digital level measurement circuitry of the decibel indication type and to telecommunication transmission systems requiring precise digital monitoring and measurement in decibels.
The invention also relates to the fields of independent voltage level measurement and to sample and hold logarithmic analog to digital converters.
2. Description of the Prior Art
Measurement of voltages and of changes in power and voltage over wide ranges of amplitude in which the total dynamic range of the parameter, such as voltage, to be measured exceeds the accuracy to which the voltage value must be known, is common in the field of electronics and in telephony related applications. An accurate and low cost digital level measurement instrument is provided by the present invention for measuring in decibels, voltage and/or power changes. By measuring in decibels the ratio either between two amounts of power, P.sub.1, and P.sub.2, or the ratio of two voltages, V.sub.1, and V.sub.2, where: EQU db= 10 log.sub.10 (P.sub.1 /P.sub.2),
and EQU db= 20 log.sub.10 (V.sub.1 /V.sub.2),
measurements over wide ranges of voltage and/or power variations may be made. In telephony, in two and four-wire switching systems noise levels are required at specified relative levels, referred to as the transmission level. Hence, a zero-transmission-level point (OTLP) is a point in a circuit with the same relative level as the sending terminals, where the sending end terminals of a long distance circuit are considered to be at a point of zero relative level. Signal level and test tones are similarly measured to monitor the performance of carrier terminal and line equipment, voice repeaters, etc. to insure that speech and other data transmissions corresponds to standardized relative levels. Further description of such standardized relative levels appears in Reference Data For Radio Engineers, Sixth Edition, Howard W. Sams Co., Inc., 1975 at 2-1 to 2-3.
Presently available analog measurement techniques are typically accurate to 0.1 decibel over a 10 decibel range with manual switching of an attenuator in ten decibel steps, which serves to range the measurement instrument over its operating span. Such measurement techniques are unsuitable for use with current programably controlled transmission equipment, which requires a digital output indicative of voltage and other electrical parameters measured to 0.01 decibel accuracy, and which can be utilized by data processing and other logic circuitry.
In the known prior art, a linear AC voltage is converted into a digital signal in decibels by the use of a logarithmic amplifier to logarithmically compress the ac signal being measured by the linear ac/dc conversion by a precision rectifier and filter. The resulting dc voltage is then translated into a digital readout proportional to the input signal in decibels by any of a number of well-known analog to digital conversion techniques.
Another technique of the known prior art in converting a linear ac voltage into a digital signal in decibels employs the technique of linear ac amplification of the signal to be measured followed by precision rectification and filtering. The filtered dc output is then logarithmically compressed using the logarithmic characteristics of a diode or one of the junctions of a transistor. A dc voltage is thus obtained which is proportional to the amplitude of the input signal in decibels and which is convertable into a digital signal by any one of a number of analog-to-digital conversion methods.
Both of the above described techniques for providing a digital decibel indication of a linear ac voltage are limited in accuracy by the stability and linearity of the logarithmic characteristic of the semiconductor device performing the conversion. Hence, without careful selection and precise temperature control of such semiconductor device, it is difficult to achieve a linearity better than .+-. 0.02 decibel over a 10 decibel dynamic range in volume production. Since the desired logarithmic current/voltage characteristic of a semiconductor is affected by other undesirable parameters, semiconductor selection and normalization is required. In the telecommunication field wherein dynamic measurement ranges of 80 db or more are required, both of the aformentioned prior art measurement circuits would be preceded in an actual measurement system with digitally controlled attenuators probably in 10 or 20 decibel steps.
Another known method of the prior art in achieving digital level measurement in decibels utilizes digitally controlled attenuators in a feedback loop whereby the amplitude of the input signal to be measured is adjusted such that after passing through the attenuator and a precision rectifier and filter, the resultant dc voltage is compared with and maintained equal to a reference dc voltage for all input signal values over the dynamic range of the measuring instrument. The attenuation value is thus a decibel representation of the input signal.
The aforedescribed measuring technique of the prior art, while capable of accuracy and linearity better than .+-.0.02 decibel over a 10 decibel dynamic range in volume production, is expensive due to the high cost of the large number of switchable attenuators required and the detrimental effect the switchable attenuators have on frequency response.
In contradistinction to the aforedescribed systems of the prior art, the present invention provides .+-.0.001 decibel linearity over a 10 decibel range and .+-.0.01 decibel linearity over a 20 decibel range without special devices or temperature dependent performance.